The present application relates to an anode and a battery including the anode.
In recent years, portable electronic devices such as combination cameras (videotape recorder), mobile phones, and notebook personal computers have been widely used, and it is strongly demanded to reduce their size and weight and to achieve their long life. Accordingly, as a power source for the portable electronic devices, a battery, in particular a light-weight secondary batter capable of providing a high energy density has been developed. Specially, a secondary battery using intercalation and deintercalation of lithium for charge and discharge reaction (so-called lithium ion secondary battery) is extremely prospective, since such a secondary battery provides a higher energy density compared to a lead battery and a nickel cadmium battery.
The lithium ion secondary battery has a cathode, an anode, and an electrolytic solution. The anode has an anode active material layer on an anode current collector. As an anode active material in the anode active material layer, a carbon material such as graphite has been widely used.
In the case that the carbon material is used as an anode active material, further improvement of the battery capacity is an issue, since the battery capacity already reaches the level close to the theoretical capacity. To solve such an issue, the following technique has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 09-204936). In the technique, by increasing the thickness of the anode active material layer, the occupancy ratio of the anode active material layer in the battery is relatively increased to the occupancy ratios of the anode current collector and the separator. In the specification, such a technique in which the thickness of the anode active material layer is intentionally increased to achieve a high capacity is referred to as “thickening of the anode active material layer.”
The technique of thickening of the anode active material layer is useful to improve the battery capacity. On the other hand, the technique causes a new issue. Specifically, when design is made so that the thickness of the anode active material layer is increased while the material and the density thereof are maintained as before under a constant battery volume, the occupancy ratio of the anode current collector in the battery is decreased, and the anode active material amount formed per unit area of the anode current collector is increased. Thus, when the same electric capacity is charged and discharged, the current density of the anode is relatively increased. Therefore, in the anode, intercalation (insertion) and deintercalation (extraction) of lithium ions are not sufficiently generated. In some cases, lithium is precipitated, becomes a dendrite, and loses its activity. In the result, input and output characteristics of the lithium ions in charge and discharge are lowered. Further, when charge and discharge are repeated, the discharge capacity is largely lowered, and thus the cycle characteristics are also lowered.
The foregoing issue similarly occurs in the case that the volume density of the anode active material layer is increased to obtain a high battery capacity as well, in addition to the case that thickening of the anode active material layer is implemented. This is because when the volume density of the anode active material layer is increased, gaps where the lithium ions are moved become small, and thus the transfer rate of the lithium ions in charge becomes slow. In the specification, such a technique to intentionally increase the volume density of the anode active material layer to achieve a high capacity is referred to as “increase of the volume density of the anode active material layer.”
In recent years, as the high performance and the multi functions of the portable electronic devices are developed, further improvement of the battery capacity is demanded. Thus, it has been considered to use silicon, tin or the like instead of the carbon material (for example, refer to U.S. Pat. No. 4,950,566). Since the theoretical capacity of silicon (4199 mAh/g) and the theoretical capacity of tin (994 mAh/g) are significantly higher than the theoretical capacity of graphite (372 mAh/g), it is prospected that the battery capacity is thereby highly improved.
When silicon or the like with the high theoretical capacity is used as an anode active material, the battery capacity is improved. On the other hand, when lithium ions are intercalated, the anode active material becomes highly activated. Thus, the electrolytic solution is easily decomposed. Accordingly, when charge and discharge are repeated, the cycle characteristics are lowered as in the case of thickening of the anode active material layer and increase of the volume density of the anode active material layer with the use of the carbon material as an anode active material.
To improve the input and output characteristics and the cycle characteristics, various techniques have been already proposed. Specifically, a technique to provide a carbon shell added with an alkali metal element or an alkali earth metal element on a crystalline graphite core in the case that a carbon material is used as an anode active material (for example, refer to Japanese Unexamined Patent Application Publication No. 2000-164218), a technique to provide a surface treatment layer containing a coating element-containing compound such as hydroxide and an electrical conductor on the surface of the anode active material (for example, refer to Japanese Unexamined Patent Application Publication No. 2003-100296), and a technique to provide a thin film made of a metal or a metal oxide on the surface of the anode active material (for example, refer to Japanese Unexamined Patent Application Publication No. 2003-249219) are known.